Eran Hammer-Lahav has put forward a proposal to also allow email addresses to be used as OpenID identifiers. Previously an OpenID identifier always was a URL or hostname (such as http://www.zefhemel.com, or more briefly zefhemel.com). I always considered this OpenID's main usability issue.
Allowing email addresses seems much more natural. In fact, many sites already do that -- all of Google's and Microsoft Password websites ask for email addresses.
The implementation is also not that complicated, it can simply be done by adding a few DNS records to the domains that are to be used by OpenID (if I wanted to use email@example.com as my OpenID I would have to add a special DNS record for the zefhemel.com domain, which would then work for all users at that domain).
I hope the proposal will be worked into the OpenID spec, it would be a great usability improvement. You can follow the discussions on the mailing list about this.